The present invention relates to a separator. The invention also relates to a battery having a separator between electrodes, a battery pack using this battery, an electronic apparatus, an electric vehicle, a power storage device, and an electric power system.
In recent years, along with the distribution of portable information-related electronic apparatuses such as mobile telephones, video cameras, and laptop computers, improvement of performance, size reduction, and weight reduction of these apparatuses have been promoted. For the power supplies of these apparatuses, disposable primary batteries or secondary batteries that can be repeatedly used are used; however, from the viewpoint of being capable of effectively achieving a comprehensive balance between enhancement of performance, size reduction, weight reduction, economic efficiency and the like, the demand for non-aqueous electrolyte batteries, particularly the demand for lithium ion secondary batteries, is increasing. Furthermore, further enhancement of performance, size reduction, and the like are underway in connection with these apparatuses, and there is also a new demand for increasing the energy density for non-aqueous electrolyte batteries such as lithium ion secondary batteries.
Thus, for the purpose of an extensive increase in the capacity of lithium ion secondary batteries, it has been suggested to use, for example, a metallic material that is alloyed with lithium at the time of charging as a negative electrode active material as described in Patent Document 1 given below, instead of the carbon-based negative electrode active materials that have been traditionally used. Specifically, silicon, tin, and compounds thereof have been suggested to be used as the metal-based negative electrode active material. For example, it is known that tin (Sn) has a high theoretical capacity (about 994 mAh/g) that highly surpasses the theoretical capacity of graphite (about 372 mAh/g) as a negative electrode active material for lithium ion secondary batteries.
On the other hand, when silicon, tin, or a compound thereof is used as a negative electrode active material, the current density per unit area is increased, and at the same time, the amount of heat generation associated with discharge tends to increase. Furthermore, in regard to the applications in electric tools, electric cars and the like, there are many occasions in which even though for a short time, heat dissipation cannot keep up with the heat generation caused by large current discharge, and there are occasions in which a temperature increase in the battery cannot be avoided. Particularly, at the time of an external short circuit or an internal short circuit of a battery, there is a risk that the amount of heat emitted from the negative electrode side is large, and the separator film is broken by this heat, so that the short circuit may be further extended, or the positive electrode is heated to reach a thermal decomposition temperature, and vigorous emission of heat or gas from the battery may occur. For this reason, the request for enhancement of reliability in a case in which large energy is emitted is also rapidly increasing, and there is a strong demand for a lithium ion secondary battery that achieves a good balance between high reliability against such a test and capacity improvement.
In this regard, it has been suggested to suppress a discharge reaction by a shutdown of the separator, or as in the case of Patent Document 2, it has been suggested to apply inorganic particles of alumina or the like on the surface of the separator. It has also been suggested to apply inorganic particles of alumina or the like on the surface of the negative electrode. Thus, there has been suggested a method of maintaining insulation between the positive electrode and the negative electrode and thereby preventing the extension of a short circuit, even in the case of abnormal heat generation that exceeds the meltdown temperature of the separator, that is, the melting point or glass transition point of the resin material that constitutes the separator.